1. Field of the Invention
This invention relates to an etchant used in a manufacturing process for semiconductor devices such as ICs and LSIs, and a surface processing method and surface processing device for silicon substrates using the above etchant, and is particularly suitable for cleaning of a silicon wafer (silicon substrate) having a silicon oxide film formed on part of the surface thereof.
2. Description of the Related Art
Generally, for cleaning silicon wafers, a cleaning solution called an SC-1 solution containing NH.sub.4 OH, H.sub.2 O.sub.2 and H.sub.2 O with the volume ratio of NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O of 1:1:5 and kept at a temperature of approx. 70.degree. C. is used.
The SC-1 solution contains ammonia which is alkaline and has a property of etching silicon. The lift-off effect obtained by etching silicon is considered as a sort of mechanism for removing particles attached to the surface of the silicon wafer. However, the SC-1 solution used in the conventional cleaning technique, the cleaning method and cleaning device using the SC-1 solution have the following problems.
(1) The surface roughness caused by etching the surface of the silicon wafer degrades the reliability of the gate oxide film. This is described in, for example, Symposium on VLSI Technology, Ohiso pp. 45-46, May 1991 M. Miyashita, M. Itano, T. Imaoka, I. Kawabe and T. Ohmi "Dependence of thin oxide films quality on surface micro-roughness".
(2) If Fe, Al, Zn or the like is contained in the SC-1 solution, the metal impurity is attached to the surface of the silicon wafer to contaminate the silicon wafer against its purpose.
(3) The etching rate by use of the SC-1 solution for silicon is different from and higher than that for silicon oxide film (Si=approx. 20 .ANG./min, SiO.sub.2 =approx. 1 .ANG./min). Therefore, the rate of removal of particles by the lift-off effect is different on the surface of the silicon wafer and on the surface of the silicon oxide film. For this reason, when the cleaning process is effected with the silicon oxide film formed on the silicon wafer, it becomes necessary to clean the silicon wafer surface and the silicon oxide film surface in different steps by use of different cleaning solutions.
(4) Since the process is effected at temperatures of 70.degree. C. to 80.degree. C., ammonia and the like are evaporated to make a contamination source in the clean room.
As described above, with the conventional etchant, and the surface processing method and surface processing device for silicon substrates using the above etchant, the surface of the processed silicon substrate becomes rough and the processing solution causes contamination by metal impurity contained therein against its purpose. Further, since the particle removing rate (etching rate) is different for the surface of the silicon substrate and for the surface of the silicon oxide film, it becomes necessary to effect different cleaning steps when the surface of the silicon substrate having a silicon oxide film formed thereon is processed, thereby increasing the number of steps. In addition, the high-temperature process is required and a problem that the cleaning solution makes a contamination source in the clean room may occur.